1. Field of the Invention
The present invention relates to proximity sensors, and, more particularly, to a tunable magnetic device for use as a proximity or position sensor.
2. Description of the Related Art
Proximity sensors are well-known devices used to detect the proximity or presence of a metallic target. The prevalent type is the xe2x80x9cinductivexe2x80x9d family of devices. These sensors react to electrically conductive materials such as steel, aluminum, copper, etc. by detecting xe2x80x9cinducedxe2x80x9d eddy-currents in the target. Although this type of device can be adapted to function well in many applications, there are several fundamental problems which limit their usefulness in the manufacturing environment.
The first type of problem encountered using an inductive sensor is the difficulty in differentiating between the target and other electrically conductive materials present. An example in an automotive assembly application would be attempting to verify the presence of a steel piston ring on an aluminum piston. What is needed is a device that can easily detect the ferrous target without being confused by the electrically conductive aluminum present in the system.
The second problem associated with inductive sensors is cost and reliability. Their complex electronic circuitry uses a relatively large number of components. The high turns count, thin wire coils required present connection and packaging reliability issues, while the magnetic cores used with these coils are costly and normally in short supply. What is needed is a lower component count, lower cost circuit and a sensing element that is more durable than the prior art tiny electromagnet and core.
The third problem encountered also relates to circuit complexity. Optimum design performance is not practical when production electronic component tolerances are taken into account. This is particularly true over extended operating temperatures. As a result, the sensor""s performance specifications must be de-rated or production yields must be lowered. Improvements in yield may be possible by xe2x80x9ctrimmingxe2x80x9d individual devices in the circuit but this is costly to implement. Recent development of xe2x80x9csmart sensorsxe2x80x9d that use integrated processors present similar problems. Their self-calibrating feature does improve sensing performance and yield however unit cost and complexity in applying these sensors in a system simply transfer problems from the manufacturer to the customer. What is needed is a non-electronic trim feature that lowers production costs while providing a wider range of performance options.
The elimination of these problems together can fulfill the needs of applications that are not easily solved today. What is therefore needed is an efficient magnetic device that incorporates either a Hall effect or magnetoresistive sensor element and signal conditioning circuitry that permits the identification of ferrous materials but not non-magnetic materials that could be electrically conductive. What is further needed is a device that uses a simple and tunable magnetic circuit that will trim out manufacturing variability.
The present invention provides a permanent magnet device for use in proximity sensing applications. The device, which may be a magnetic circuit, utilizes a magnetic field sensor which has either an analog or digital output. The device provides a magnetic field that is sensitive to the proximity of either ferrous or permanent magnet materials. Further, the device provides a method to balance the magnetic field locally where the field sensor exists. The device overcomes the shortcomings of the prior art by providing an adjustable proximity sensing device.
A wide range of applications exist for tunable magnetic devices which employ magnetic field sensors, including position monitoring, proximity sensing, and counting applications.
The present invention, in one form thereof, comprises a tunable magnetic device. A permanent magnet in the shape of a ring or block is provided with an inner aperture. A magnetic field sensor is located above the inner aperture and is attached to the permanent magnet. A ferrous tuning device is located within the inner aperture and beneath the field sensor. The permanent magnet provides a bias magnetic field. The ferrous tuning device provides a magnetic field in opposition to the field produced by the permanent magnet. In essence, the ferrous tuning device provides a deductive magnetic field with respect to the field provided by the permanent magnet. The ferrous tuning device is movable relative to the permanent magnet and the field sensor. Adjustment of the location of the ferrous tuning device affects the magnetic field to which the field sensor is exposed. By adjusting the location of the ferrous tuning device, a desired bias field level can be obtained. This field adjustment can trim the field sensor for a number of functions or application requirements.
The present invention, in another form thereof, comprises a tunable magnetic device. A permanent magnet in the shape of a ring is axially magnetized to bias the magnetic sensor. An inner magnet located concentrically within the permanent magnet is axially magnetized in the direction opposite of the permanent magnet, thereby acting as a deductive magnet with respect to the field of the permanent magnet. A magnetic field sensor is located above the inner magnet and is attached to the permanent magnet. The inner magnet is movable relative to the permanent magnet and the field sensor. Adjustment of the location of the inner magnet affects the magnetic field to which the field sensor is exposed.
The present invention, in still another form thereof, comprises a device having a tunable magnetic circuit that is made up of a ring-shaped permanent magnet which is axially magnetized to bias a magnetic sensor. An inner magnet is located concentrically within the permanent magnet. The inner magnet is axially magnetized in a direction opposite of the outer ring magnet. A field sensor is located above the inner magnet. The inner magnet can be adjusted in magnetic potential by the use of an external field. This external field can be used to either raise or lower the magnetic potential of the inner magnet so that the complete magnetic circuit is tuned for proper performance.
The invention, in yet another form thereof, comprises a device having a tunable magnetic circuit that is made up of a single permanent magnet which is magnetized with concentric poles. The inner magnetic pole is located concentrically within the permanent magnet. The inner magnetic pole magnetized in a direction opposite of the outer ring magnet. A field sensor is located above the inner magnetic pole. The inner magnet can be adjusted in magnetic potential by the use of an external field. This external field can be used to either raise or lower the magnetic potential of the inner magnetic pole so that the complete magnetic circuit is tuned for proper performance.
In operation, a ferrous object brought in proximity to the tunable device will have the greatest influence on the outer permanent magnet and thereby imbalance the tuned magnetic field. This imbalance is measured by the magnetic field sensor. Besides ferrous objects, permanent magnet objects will also imbalance the tuned magnetic field as measured by the magnetic field sensor.
An advantage of a proximity sensing device according to the present invention is the simplicity of construction.
Another advantage of the present invention is the provision of a tuning mechanism which permits adjustment after device assembly.
A further advantage of the present invention is that outreach of the magnetic flux is greater thereby providing less critical spacing of the ferrous object to be sensed.
Another advantage of the present invention is the efficient detection of ferrous and permanent magnet objects.
Yet another advantage of the present invention is the ability to null out application magnetic fields where necessary.
A still further advantage of the present invention is the ability to trim the field sensor for a particular duty cycle.
Another advantage of the present invention is the ability to adjust for device offset requirements.
Yet a further advantage of the present invention is the ability to trim the field sensor for a specific operation when a target is at a desired position.